I'm entering this as a bug because it feels like a very significant regression to me, in my usage pattern.
I add quite a few books, and "add by ISBN" seldom works for me. It was a minor nuisance but at least when it did fail, I used to get a message on my screen immediately that said "We were unable to import your book:
You can add the book manually by clicking the link below."
Last night, I was perplexed when 2 attempts to add books by ISBN failed with ZERO visible sign of failure. In one case, I tried 3 times, the other twice, because I did not know what was happening.
Then, just now, I noticed the red notification dot on my profile pic and saw two notifications for the failed imports. Because these notifications are now 10 hours old, I have to search the ISBNs to find out which books I was trying to add.
The old error message was much more visible and made it easier for me to manually add the book while the relevant external info pages were sill open, I hope some thought can be given to restoring that behaviour. Although it is probably less likely to happen going forward as I have bookmarked the "add manually" page to avoid wasting time trying to import by ISBN first.
This is still happening, and importing books continues to be the major pain point in my TSG experience. Five books added in one hour with three different failure types. The one thing I truly dread is adding books to my TBR, sadly.
The "failure" (more the lack of notification explaining what was happening!) is no longer on silent on Firefox (which is what was happening here)!
This change came with an improvement to our import book system. It was added because, on average, we're now able to import more books for people over a longer period of time. Fewer people get an immediate failure. Some have to wait longer, but then they'll get a notification saying their book has been added.
Regarding the failure being "silent" — what type of device are you using? Have you seen the notification that pops up that says: "We're still trying to import your book. We'll send you an in-app notification when it's ready."? It's in exactly the same design as the old error message — just the text is different!
And when you see it, you're still free to add in the book yourself immediately after. On the ISBN import page there's a direct link to the manual add form.
Separate to that, the books you're trying to import do seem to have a higher proportion of failure and we're looking into how we can reduce that. That way, you'll either get more immediate success or links to your books some hours later, making for an experience which requires you to do less work.
Aaand — I've just tested this on Firefox and seen that the notifications do not pop up! This is likely what has been contributing to your poor experience. I'll look into this today.